1. Field of the Invention
This invention relates generally to a system for performing computed x-ray tomography and more particularly to an arrangement for precisely indicating the proper staged position of a patient on a table prior to performing the tomography.
2. Description of the Prior Art
In one method of computed tomography a patient is supported for being translated along a longitudinal axis which is usually horizontally disposed. The axis coincides with the center of rotation of a rotatable gantry which has an x-ray source on one side of the center of rotation and a multiple array of x-ray detectors on the other side. A fan shaped x-ray beam that is thin in the longitudinal direction is projected through the patient as the gantry rotates so that the detectors may develop signals indicative of x-ray transmission characteristics along a plurality of paths through the patient undergoing examination. Analog signals representative of x-ray attenuation by all of the volume elements in a layer of the patient at various rotational angles are then converted to digital signals which are used by a computer to produce signals which may thereafter be used to produce a reconstructed image of the layer. The image is reconstructed by analyzing all of the signals within a central cross-sectional area of the system known as the "reconstruction circle". The analog signals are analyzed and converted based upon all of the x-ray attenuation taking place within the reconstruction circle. If the patient is improperly positioned so that a portion of the patient extends outside of the reconstruction circle, the attenuation of that portion will be erroneously interpreted. It is therefore important that the patient be properly aligned within the center of the reconstruction circle during the scan.
The x-ray beam is usually projected through a thin layer of the patient so that the reconstructed layer is essentially perpendicular to the longitudinal axis along which the patient is translated to obtain views of successive layers.
In the above described system, the patient is supported on a horizontal table which is translatable along a central longitudinal axis. The patient is positioned on the table and generally aligned for the first or reference scan. It is extremely important to determine the precise location of the first scan of the patient. By knowing the precise location of the first scan, the number of lead-in scans prior to the area of interest and lead-out scans following the area of interest can be eliminated and thereby maximize the utilization of the computed tomography system and minimize the radiation exposure to the patient. Knowledge of the reference scan is also required for analysis and correlation of the reconstructed images of the patient. As previously discussed, axial alignment is also necessary to facilitate centering of the patient within the image reconstruction circle of the system and also to facilitate analysis of images having symmetrical patterns.
One method of aligning and positioning a patient to a specific first scan involves the actual translation of the patient into the gantry prior to performing the scanning operation. Even with the help of internal light beams it is very difficult to observe the position of the patient relative to the geometry of the gantry and also difficult to reposition the patient while in the translated position. It is often necessary to retract the table for patient repositioning and then repeat the verification operation.
Another problem associated with patient positioning involves scans of which the gantry is tilted through a small vertical angle and is not perpendicular to the horizontal or longitudinal axis. On some occasions unique diagnostic information can only be obtained by viewing a slice or layer through the body which is taken at such a small vertical angle and is not perpendicular to the longitudinal axis along which the patient is translated. A tomography system having such a tiltable gantry is described in U.S. Pat. application Ser. No. 771,863 entitled GANTRY FOR COMPUTED TOMOGRAPHY filed in the name of Kelman et al and assigned to the same assignee as the present application. As the gantry is tilted either rearwardly (cranially) or forwardly (caudally), a reference line perpendicular to the top surface of the patient would not accurately display the reference scan.
Another problem associated with patient positioning involves a scan by a vertical gantry while the patient is tilted to a specific angle; particularly for neurological analysis of the head. One method of positioning the patient to the desired specific angle while on a horizontal table requires the use of a set of transparent protractor devices to be aligned with inscribed lines on the surface of the patient. These devices are time consuming, difficult to use and are not extremely accurate. Alternatively, the angular orientation can be accomplished by tilting the table through a small horizontal angle relative to the longitudinal axis of a vertical gantry. As the table is tiled, a reference line on the patient would not accurately display the horizontal alignment of the patient. Also, the horizontal translation of the angled table into the gantry may be less than when in the horizontal position and a reference line perpendicular to the top surface of the patient may not accurately display the reference scan.
Accordingly, one object of this invention is to provide an arrangement to indicate the position of the patient relative to the first scan of a gantry while the patient is in the staged position.
Another object of the present invention is to provide an arrangement to facilitate the alignment of a patient on a table while in the staged position of a computed tomography system.
Another object is to provide an arrangement for accurately indicating the plane of the first scan of a computed tomography system when the gantry of the system is tilted through a small vertical angle and is not perpendicular to the horizontal or longitudinal axis along which the patient is translated.
Still another object of this invention is to provide an arrangement for displaying a desired angle on the surface of a patient so that the patient can be easily and accurately positioned at that desired angle while in the staged position prior to translation into the gantry for scanning.